When a motion picture is shown at a concert hall and a movie theater, an audio signal is generally reproduced by means of Dolby surround-phonic sound system 4-channel stereo reproducing system to urge audiences to feel as if they were in the very place shown on the picture screen.
When a motion picture for a movie theater is produced, a video signal is recorded and at the same time, left front sound, right front sound, center front sound and rear sound are recorded respectively. In this case, a left front signal L.sub.F, a right front signal R.sub.F, a center front signal F and a rear signal B are encoded respectively and then recorded on a video disc or a magnetic tape as LT and RT signals, respectively. In this case, in order to improve a signal-to-noise ratio, or a so-called S/N ratio of the rear signal B, there is employed a so-called modified B-type Dolby system in which a high frequency component higher than 1 kHz is emphasized (pre-emphasis) by a predetermined amount and then recorded. FIG. 3 illustrates frequency vs. response characteristics in the case where the recording level is at 0 dB, -10 dB, -20 dB and -30 dB in this modified B-type Dolby system. Then, the left front signal L.sub.F, the right front signal R.sub.F, the center front signal F and the rear signal B thus recorded are further encoded to a left composite signal L.sub.T and a right composite signal R.sub.T which are respectively expressed as L.sub.T =L.sub.F +(F/2+2) and R.sub.T =R.sub.F +(F/2)-(B/2). These left and right composite signals L.sub.T and R.sub.T are optically recorded, in the case of, for example, a 35 mm film, at a sound track portion located at a predetermined position of the film according to the brightness change system which carries out the recording on the basis of the change of light and shade of, for example, the coloring. Accordingly, when a motion picture is shown, the left and right composite signals L.sub.T and R.sub.T thus optically recorded are decoded so as to produce four signals of the left composite signal L.sub.T, the right composite signal R.sub.T, a sum signal L.sub.T +R.sub.T of the left composite signal L.sub.T and the right composite signal R.sub.T and a difference signal L.sub.T -R.sub.T of the left composite signal L.sub.T and the right composite signal R.sub.T. Then, these four signals are reproduced by a left front loudspeaker, a right front loudspeaker, a center front loudspeaker and a rear loudspeaker. In this case, since the high frequency component of the rear signal B is pre-emphasized by the modified B-type Dolby system as described hereinabove, upon reproducing, the high frequency component of the difference signal L.sub.T -R.sub.T is attenuated (de-emphasized) so as to make a total frequency characteristic flat relative to the rear signal B. When the audio signal is reproduced by such 4-channel stereo reproducing system, the sound effect is enhanced to provide presence so that the listeners are urged to feel as if they were in the very place shown on the picture screen.
By the way, a magnetic tape and a video disc by which the user can enjoy the screen music recorded thereon for the motion picture or the mortion picture itself are now available on the market. In this case, since the recorded signals recorded on the magnetic tape and the video disc are the composite signals L.sub.T and R.sub.T as described hereinabove, there is proposed a multi-channel stereo reproducing apparatus illustrated in FIG. 1.
In FIG. 1, reference numerals 1 and 2 respectively designate left and right composite signal input terminals to which the left and right composite signals L.sub.T and R.sub.T are supplied. The respective left and right composite signal input terminals 1 and 2 are connected to the input side of a balancing circuit 3, and the balancing circuit 3 produces at its output sides left and right composite signals L.sub.T and R.sub.T which are so balanced as to localize an acoustic image normally.
The output sides of the balancing circuit 3 are connected through amplifiers 4 and 5 to left and right front loudspeakers 6 and 7 which respectively reproduce the left and right composite signals L.sub.T and R.sub.T supplied to the left and right composite signal input terminals 1 and 2.
Also, the output sides of the balancing circuit 3 are connected to the input side of a mixing circuit 8 which generates at its output side the sum signal L.sub.T +R.sub.T of the left and right composite signals L.sub.T and R.sub.T. The output side of the mixing circuit 8 is connected through an amplifying circuit 9 to a center front loudspeaker 10 which reproduces the sum signal L.sub.T +R.sub.T developed at the output side of the mixing circuit 8.
Further, the output sides of the balancing circuit 3 are connected to the input sides of a subtracting circuit 11 and the subtracting circuit 11 generates at its output side the left and right composite signals L.sub.T and R.sub.T of which the monaural components are removed. The output side of the subtracting circuit 11 is connected to the input side of a delay circuit 12, and the delay circuit 12 generates at its output side a delayed difference signal .DELTA.t (L.sub.T -R.sub.T) which is delayed by a predetermined delay time. Thus, a front localization feeling and a widening feeling of sound field are obtained. The output side of the delay circuit 12 is connected to the input side of a filter for passing therethrough a signal of low frequency band, that is, so-called low pass filter 13, and the low pass filter 13 generates at its output side the delayed difference signal .DELTA.t (L.sub.T -R.sub.T) in which a clock pulse component given by the passing of the signal through the delay circuit 12 and the like are removed and the high frequency component higher than a predetermined frequency is removed so as to balance the low frequency component which is not sufficient in the difference signal L.sub.T -R.sub.T. The output side of the low pass filter 13 is connected to the input side of a de-emphasis circuit 14 which attenuates the high frequency component of the delayed difference signal .DELTA.t (L.sub.T -R.sub.T) higher than 1 kHz, thus producing a delayed difference signal .DELTA.t (L.sub.T -R.sub.T) in which the total frequency characteristic of the rear ? signal B of which the high frequency component higher than 1 kHz is emphasized in response to the response thereof is made flat. Then, the output side of the de-emphasis circuit 14 is connected through amplifying circuits 15 and 16 to left and right rear loudspeakers 17 and 18 by which the delayed difference signal .DELTA.t (L.sub.T -R.sub.T) in which the T frequency characteristic of the rear signal B is made flat, is reproduced.
In the conventional multi-channel stereo reproducing apparatus thus constructed, the left and right composite signals L.sub.T and R.sub.T are reproduced from the left and right front loudspeakers 6 and 7, the sum signal L.sub.T +R.sub.T is reproduced from the center front loudspeaker 10 and the delayed difference signal .DELTA.t (L.sub.T -R.sub.T) is reproduced from the left and right rear loudspeakers 17 and 18.
Consequently, according to such known multi-channel stereo reproducing apparatus, the sound effect is enhanced to produce good stereo presence the same as that in the concert hall.
However, by such conventional multi-channel stereo reproducing apparatus, when reproducing a recording medium, such as, a record disk, a cassette tape and so on available on the market and on which the standard 2-channel stereo signal formed of the left front signal L.sub.F and the right front signal R.sub.F in which the center front signal F and the rear signal B are not encoded at all are recorded, since the delayed difference signal .DELTA.t (L.sub.F -R.sub.F) from the delay circuit 12 passes through the de-emphasis circuit 14, its high frequency component higher than 1 kHz is attenuated. Then, the delayed difference signal .DELTA.t (L.sub.F -R.sub.F) of which the high frequency component is attenuated is reproduced from the left and right rear loudspeakers 17 and 18, so that the high frequency component of the sound reproduced by the rear loudspeakers is attenuated unnaturally. Thus, satisfactory stereo presence can not be obtained.